Hello World - Spring Boot Java

This guide describes the steps required to create the helloworld-java-spring sample app and deploy it to your cluster.

The sample app reads a TARGET environment variable, and prints Hello ${TARGET}!. If TARGET is not specified, World is used as the default value.

You can also download a working copy of the sample, by running the following commands:

git clone -b "main" https://github.com/knative/docs knative-docs
cd knative-docs/docs/serving/samples/hello-world/helloworld-java-spring

Prerequisites

  • A Kubernetes cluster with Knative installed and DNS configured. Follow the installation instructions.
  • Docker installed and running on your local machine, and a Docker Hub account configured.
  • (optional) The Knative CLI client kn can be used to simplify the deployment. Alternatively, you can use kubectl, and apply resource files directly.

Building the sample app

  1. From the console, create a new, empty web project by using the curl and unzip commands:

    curl https://start.spring.io/starter.zip \
    -d dependencies=web \
    -d name=helloworld \
    -d artifactId=helloworld \
    -o helloworld.zip
unzip helloworld.zip

    If you don’t have curl installed, you can accomplish the same by visiting the Spring Initializr page. Specify Artifact as helloworld and add the Web dependency. Then click Generate Project, download and unzip the sample archive.

  2. Update the SpringBootApplication class in src/main/java/com/example/helloworld/HelloworldApplication.java by adding a @RestController to handle the “/” mapping and also add a @Value field to provide the TARGET environment variable:

    package com.example.helloworld;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@SpringBootApplication
public class HelloworldApplication {

  @Value("${TARGET:World}")
  String target;

  @RestController
  class HelloworldController {
    @GetMapping("/")
    String hello() {
      return "Hello " + target + "!";
    }
  }

  public static void main(String[] args) {
    SpringApplication.run(HelloworldApplication.class, args);
  }
}

  3. Run the application locally:

    ./mvnw package && java -jar target/helloworld-0.0.1-SNAPSHOT.jar

    Go to http://localhost:8080/ to see your Hello World! message.

  4. In your project directory, create a file named Dockerfile and copy the code block below into it:

    # Use the official maven/Java 8 image to create a build artifact.
# https://hub.docker.com/_/maven
FROM maven:3.5-jdk-8-alpine as builder

# Copy local code to the container image.
WORKDIR /app
COPY pom.xml .
COPY src ./src

# Build a release artifact.
RUN mvn package -DskipTests

# Use AdoptOpenJDK for base image.
# It's important to use OpenJDK 8u191 or above that has container support enabled.
# https://hub.docker.com/r/adoptopenjdk/openjdk8
# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
FROM adoptopenjdk/openjdk8:jdk8u202-b08-alpine-slim

# Copy the jar to the production image from the builder stage.
COPY --from=builder /app/target/helloworld-*.jar /helloworld.jar

# Run the web service on container startup.
CMD ["java", "-Djava.security.egd=file:/dev/./urandom", "-jar", "/helloworld.jar"]

For detailed instructions on dockerizing a Spring Boot app, see Spring Boot with Docker.

For additional information on multi-stage docker builds for Java see Creating Smaller Java Image using Docker Multi-stage Build.

NOTE: Use Docker to build the sample code into a container. To build and push with Docker Hub, run these commands replacing {username} with your Docker Hub username.

  1. Use Docker to build the sample code into a container, then push the container to the Docker registry:

    # Build the container on your local machine
docker build -t {username}/helloworld-java-spring .

# Push the container to docker registry
docker push {username}/helloworld-java-spring

Deploying the app

After the build has completed and the container is pushed to Docker Hub, you can deploy the app into your cluster.

  1. Create a new file, service.yaml and copy the following service definition into the file. Make sure to replace {username} with your Docker Hub username.

    apiVersion: serving.knative.dev/v1
kind: Service
metadata:
  name: helloworld-java-spring
  namespace: default
spec:
  template:
    spec:
      containers:
        - image: docker.io/{username}/helloworld-java-spring
          env:
            - name: TARGET
              value: "Java Spring Sample v1"

Ensure that the container image value in service.yaml matches the container you built in the previous step. Apply the configuration using kubectl:

kubectl apply --filename service.yaml

With kn you can deploy the service with

kn service create helloworld-java-spring --image=docker.io/{username}/helloworld-java-spring --env TARGET="Java Spring Sample v1"

This will wait until your service is deployed and ready, and ultimately it will print the URL through which you can access the service.

During the creation of your service, Knative performs the following steps:

  • Create a new immutable revision for this version of the app.
  • Network programming to create a route, ingress, service, and load balance for your app.
  • Automatically scale your pods up and down, including scaling down to zero active pods.

Verification

  1. Find the domain URL for your service:

    kubectl get ksvc helloworld-java-spring  --output=custom-columns=NAME:.metadata.name,URL:.status.url

    Example:

    NAME                      URL
helloworld-java-spring    http://helloworld-java-spring.default.1.2.3.4.xip.io

    kn service describe helloworld-java-spring -o url

    Example:

    http://helloworld-java-spring.default.1.2.3.4.xip.io

  2. Make a request to your app and observe the result. Replace the URL below with the URL returned in the previous command.

    Example:

    curl http://helloworld-java-spring.default.1.2.3.4.xip.io
Hello Java Spring Sample v1!

# Even easier with kn:
curl $(kn service describe helloworld-java-spring -o url)

    Note: Add -v option to get more detail if the curl command failed.

Removing

To remove the sample app from your cluster, delete the service record.

kubectl delete --filename service.yaml

kn service delete helloworld-java-spring